Method of and electrolyte for



Patented Jan. 12, 1937 TATES METHOD OF AND ELECTROLYTE' FOR DEPOSITINGBHODIUM Christian William Keitel, West Orange, N. J assignor to Baker &Co. Inc., Newark, N. J., a corporation of New Jersey No Drawing.Application September 19, 1931,

' Serial No. 563,885

13 Claims.

This invention relates to an electrolytev for electrodepositing rhodiumand the method of using the same.

One object of my invention is the provision of an acid electrolyte whichproduces very white deposits when used for electrodepositing rhodium.

Another object of this invention is the provision, of an electrolytewhich may be .used to form a tarnish resisting coating on metal surfacessuch as silver or the like, which are not tarnish resisting bythemselves, and whereby the coating is of such a-whiteness that itcannot readily be distinguished from silver.

Further objects and advantages of my invention will be in part obviousand in part specifically pointed out in the description hereinaftercontained, which discloses one embodiment of my invention, it beingexpressly understood, however, that various changes may be made inpractice within the scope of the appended claims without digressing frommy inventive idea.

According to my invention, a rhodium bath may be prepared in thefollowing manner. Two grams of rhodium in the form of rhodium ammoniumnitrite are suspended in 1000 cc. of 5% sulphuric acid, and the mixtureis heated until solution is complete. The rhodium ammonium nitrite is avery insoluble salt, and the heating must be continued for a substantiallength of time. given off, and the rhodium ammonium nitrite isapparently partially decomposed. When solution is complete, the bath iscooled until the temperature is about 50 C. and is used as anelectrolyte. A voltage of about 4 volts and a current density of aboutto amperes per square decimeter are used.

The above proportions are merely given for the purpose of illustrationof one form of my invention, and are not to be construed as limiting myinvention to these proportions. Likewise the temperature and the voltageare not to be considered as invariable, as they may be varied, and theadvantages set forth in connection with my invention may be obtained.

Another method of preparing my electrolyte is to boil rhodium ammoniumnitrite in distilled water until the salt goes in solution. The heatingtakes about 10 to 30 hours. This solution contains a water soluble acidrhodium ammonium nitrite compound. The compound is a soluble alkalirhodium nitrite compound. Then prepare a solution of 50 cc. ofconcentrated sulphuric acid in one liter of distilled water and add 2grams of rhodium in the form of the aqueous solution During the heating,brown fumes areprepared as above indicated. This bath is then used as anelectrolyte for depositing rhodium at a temperature of about 50 C. Thevoltage used is about 4 volts and the current density is about 20 to 30amperes per square decimeter.

My invention is not restricted to rhodium ammonium nitrite as otherinsoluble alkali rhodium nitrites may be used, as, for example, rhodiumpotassium nitrite .and rhodium sodium ammonium nitrite. These insolublecompounds are rendered soluble when treated in the manner abovedescribed in connection with rhodium ammonium nitrite.

In replenishing the bath it is only necessary to add rhodium ammoniumnitrite in a quantity sufficient to bring the bath up to 2 grams ofrhodium per liter, repeating either of the above described processes ofbringing the rhodium salt in solution.

The electrolytes made according to my invention are used principally fortarnish proofing 20 metals which are not tarnish resisting bythemselves, but may also be used for improving the color of preciousmetals. When a deposit of rhodium is made, the color of the depositapproximates the color of silver very closely, and whiter 25 depositsare obtained than the deposits heretofore obtained from electrolyteswhich are neutral or ammoniacal.

To my acid electrolytes I may add different conducting salts, as, forexample, sodium sulphate, or the like. However, it is not absolutelynecessary to add any conducting salts to my electrolyte. Other inorganicacids, such as nitric acids and also organic acids, such as citric,tartaric, oxalic, or chloracetic acid, may beused instead of thesulphuric acid, or admixed therewith.

By electrodepositing rhodium from my electrolytes, it is possible toobtain a coating which tarnish proofs jewelry articles, optical, frames,etc., the deposits being whiter than the deposits heretofore produced.

In some instances it may be desirable to deposit a flash coating orheavy coating of another metal on the silver article to be tarnishproofed before depositing the rhodium coating. I have found, forinstance, that in certain cases a copper deposit under the rhodiumallowed me to reduce the rhodium deposits to a minimum without beingdetrimental to the length of the life of the tarnish proofing. Othermetals might be used for under plating, and I do not wish to limitmyself in this respect.

Although particular and preferred forms of my invention have beendescribed, it is to be expressly understood that various changes may bemade in practice within, the-scope of the claims without digressing frommy'inventive idea.

What I claim is:

1. An electrolyte for depositing rhodium, made by dissolving rhodiumammonium nitrite in an acid solution.

2. An electrolyte for depositing rhodium, made by dissolving a normallywater insoluble alkali metal ,(includingl ammonium) rhodium nitritecompound in sulphuric acid.

3. An electrolyte for depositing rhodium, which comprises, the productobtained by dissolving rhodium ammonium nitrite in an acid solutioncontaining sulphuric acid.

-4. Am electrolyte adapted for electrodeposit 8. The method ofelectrodepositing rhodium,

which comprises, passing an electric current through an electrolyteproduced by dissolving rhodium ammonium nitrite in sulphuric acid.

9. The method of electrodepositing rhodium, which comprises, passing anelectric current through an electrolyte containing an acid and apartially decomposed normally water insoluble alkali metal (includingammonium) rhodium nitrite compound.

10. The method of tarnish-proofing silver or the like, which comprises,immersing the article to be tarnish-proofed as the cathode in anelecolyte produced by dissolving a normally water insoiuble alkali metal(including ammonium) rhodium nitrite in an acid solution and passing anelectric current through the same.

11. The method of tarnish-proofing silver or the like, which comprises,immersing the article to be tarnish-proofed in an electrolyte containingan acid and a partially decomposed normally water insoluble alkali metal(including ammonium)'\ rhodium nitrite and passing a current through thesame.

12. An electrolyte adapted for electrodpositing rhodium, which containsan inorganic acid and a water soluble partially decomposednormally waterinsoluble alkali metal (including ammonium) rhodium nitrite compound.

. 13.- An electrolyte adapted for'electrodepositing rhodium, whichcontains sulphuric acid and a water soluble partially decomposednormally water insoluble alkali metal (including ammonium) rhodiumnitrite compound.

CHRISTIAN WILLIAM KEITEL.

